Until the mid-1980s, the accepted treatment for atherosclerosis, i.e., narrowing of the coronary artery(ies) was coronary by-pass surgery. While effective and while having evolved to a relatively high degree of safety for such an invasive procedure, by-pass surgery still involves potentially serious complications that, in the best of cases, require an extended recovery period.
With the advent of percutaneous tranluminal coronary angioplasty (PTCA) in 1977, the scene changed dramatically. Using catheter techniques originally developed for heart exploration, inflatable balloons were employed to re-open occluded regions in arteries. The procedure was relatively non-invasive, took a very short time compared to by-pass surgery and the recovery time was minimal. However, PTCA brought with it other problems such as vasospasm and elastic recoil of the stretched arterial wall which could undo much of what was accomplished and, in addition, it created a new disease, restenosis, the re-clogging of the treated artery due to neointimal hyperplasia.
The next improvement, advanced in the mid-1980s was the use of a stent to maintain the luminal diameter after PTCA. This for all intents and purposes put an end to vasospasm and elastic recoil but did not entirely resolve the issue of restenosis. That is, prior to the introduction of stents, restenosis occurred in from 30-50% of patients undergoing PTCA. Stenting reduced this to about 15-20%, much improved but still more than desirable.
In 2003, drug-eluting stents or DESs were introduced. The drugs initially employed with the DES were cytostatic compounds, compounds that curtailed the proliferation of cells that resulted in restenosis. The occurrence of restenosis was thereby reduced to about 5-7%, a relatively acceptable figure. Today, the DES is the default the industry standard to treatment of atherosclerosis and is rapidly gaining favor for treatment of stenoses of blood vessels other than coronary arteries such as peripheral angioplasty of the femoral artery.
There are several characteristics that are important for implantable medical devices including high radial strength, good fracture toughness, resistance to creep, low stress relaxation, and minimal physical aging. Creep refers to the gradual deformation that occurs in a polymeric construct subjected to an applied load. Creep, for example, can result in an expanded stent retracting radially inward, reducing the effectiveness of a stent in maintaining vascular patency.
There is, therefore, an on-going need for implantable medical devices made of polymers that more completely meet the above criteria. The current invention provides a composition that meets the indicated criteria and implantable medical devices made therefrom.